syndrome in the first trimester but is usually not performed until after the beginning of the second trimester, after the 14th week of pregnancy. Amniocentesis involves inserting a needle into the uterus to withdraw fetal cells from the amniotic fluid, for later chromosomal analysis. The procedure carries with it a 0.5 percent risk of miscarriage.

Because of the invasiveness, risk, and cost of cvs and amniocentesis, scientists have sought ways to screen pregnancies with simpler tests that would provide an indication of women with a higher liklihood of carrying a fetus with Down syndrome. These higher risk women could then be offered the diagnostic procedures of CVS or amniocentesis. Currently, screening for Down syndrome takes place during the second trimester of pregnancy and involves testing for alpha-fetoprotein, unconjugated estriol, as well as human chorionic gonadotropin. This study extends screening to the first trimester.

Women with abnormal levels of these substances are usually then referred for diagnosis by amniocentesis. For the study, Dr. Haddow and his coworkers collected blood samples from a total of 4,412 pregnant women at 16 prenatal diagnostic centers in the United States. The researchers then analyzed the samples for various substances often found in abnormal levels in the blood of women pregnant with a fetus with Down syndrome. Among the substances for which the researchers tested were: human chorionic gonadotropin (hCG); a portion of the hCG molecule, known as the free beta subunit of hCG; and pregnancy-associated protein A. Smaller studies conducted previously have shown that both hCG levels and levels of the beta subunit of hCG are higher in the presence of a fetus with Down syndrome than in a fetus with a normal chromosome count. Conversely, blood levels of protein A are lower than normal in the presence of a fetus with Down syndrome.

For the women taking part in the study, a total of 48 pregnanc
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Contact: Bob Bock
rb96a@nih.gov
301-496-5133
NIH/National Institute of Child Health and Human Development
1-Apr-1998

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